1. Field of the Invention
The present invention relates to a charging circuit for a frequency converter, wherein a first constant current source in the first feed line to an intermediate circuit capacitor limits the charging circuit of the intermediate circuit capacitor.
2. Discussion of Related Art
Such charging circuits are required for dependably charging the large capacitance existing in the intermediate circuit of a capacitor prior to the actual current supply for the capacitor being turned on.
The development of modern frequency converters permits the effective employment of electrical drive mechanisms in increasingly expanding areas of technology. By first rectifying the available supply net voltage, and thereafter processing it by an inverter for controlling a motor, it is possible to realize very accurate control systems for the position or the speed of a drive mechanism. This finds important applications in machine tools, for example. Following the rectification of the supply net voltage, it is customary to perform the smoothing of the voltage and storage of energy by means of an intermediate circuit capacitor of high capacitance in the so-called intermediate circuit. When putting the frequency converter into operation, it is necessary to first charge such an intermediate circuit capacitor, and a special charging circuit is often used for this. In order to not let the charging current of the intermediate circuit capacitor become too high, a limiting device that limits the charging current is necessary. For example, simple resistors, or also constant current sources, can be used.
DE 101 48 740 describes how a high charging current can arise in spite of such a current limitation by a resistor or a constant current source. If, for example, a line-to-ground fault exists at the output of the frequency converter, for example in the connected motor, a very high charging current can flow over one of the recovery diodes of the inverter and cause damage to the frequency converter. Although this high current can only flow as long as the intermediate circuit capacitor is not charged, the high capacitance of the intermediate circuit capacitor permits currents having damaging effects. A circuitry is therefore described in DE 101 48 740, which can detect a line-to-ground fault to begin with and prevents the start-up of the frequency converter in such a case.
However, since it is easily possible by a higher control device to pinpoint such a line-to-ground fault more accurately, for diagnostic purposes the start-up of the frequency converter is desirable in spite of a detected line-to-ground fault. A charging circuit is necessary for this, which permits the charging of the intermediate circuit capacitor with a limited current in spite of a line-to-ground fault at the output of the frequency converter. Once the intermediate circuit capacitor has been charged, the above mentioned error mechanism is no longer of importance, it is now possible on the part of the control device to perform a diagnostic program for pinpointing the line-to-ground fault.